Room-temperature ionic liquids (RTILs) are organic salts that are liquid at or below 100° C., and composed entirely of cations and anions (free of additional solvents) (Welton, 1999, Chem. Rev. 99:2071-2083; Welton, 2004, Coord. Chem. Rev. 248:2459-2477). They are useful as solvents and liquid media due to their unique properties: very low volatility, relatively low viscosity, high thermal stability, low flammability, high ionic conductivity, tunable polar solvation and transport properties, and in some cases, even catalytic properties. These characteristics have made RTILs excellent candidates as a more environmentally benign class of solvents to replace conventional organic solvents in chemical, electrochemical, and physical extraction/separation processes. In addition, RTILs are useful as novel gas separation media in supported liquid membranes (SLMs) and novel catalysts in chemical processes (Scovazzo et al., “Supported Ionic Liquid Membranes and Facilitated Ionic Liquid Membranes,” ACS Symposium Series 818 (Ionic Liquids), 2002, 69-87; Schaefer et al., “Opportunities for Membrane Separation Processes using Ionic Liquids,” ACS Symposium Series 902 (Ionic Liquids IIIB: Fundamentals, Progress, Challenges, and Opportunities), 2005, 97-110; Riisager & Fehrmann, Ionic Liquids in Synthesis (2nd ed.), Wiley-VCH: Weinheim, Germany, 2007; 527-558; Scovazzo et al., 2004, J. Mem. Sci. 238:57-63; Jiang et al., 2007, J. Phys. Chem. B. 111:5058-5061). Research in recent years of RTILs as selective gas separation media has focused on CO2-based separations, with SO2 removal also appearing to be a promising pursuit (Jiang et al., 2007, Phys. Chem. B 111:5058; Huang et al., 2006, Chem. Commun. 38:4027; Anderson et al., 2006, J. Phys. Chem. B 110:15059).
Effective and economical removal of CO2 from process streams containing other light gases, such as N2, CH4, or H2, is of vital importance and represents an ongoing chemical engineering challenge (“Basic Research Needs for Geosciences: Facilitating 21st Century Energy Systems,” Office of Basic Energy Sciences of the U.S. Dept. of Energy, 2007; Jones & Maginn, 2010, ChemSusChem 3:863-864; Descamps et al., 2008, Energy 33:874-881; Sridhar et al., 2007, Sep. Purif. Rev. 36:113-174; Review of emerging resources: U.S. Shale gas and shale plays, 2011; available from: http://www dot eia dot gov). Specifically, the separation of CO2 from N2, from CH4, and from H2 are three distinct separation challenges faced by the electrical energy, natural gas, and syngas production sectors, respectively. In the production of electrical energy, potential climate change issues attributed to anthropogenic CO2 have recently highlighted the importance of CO2 separation from flue gas (i.e., CO2/N2 separation) (“Basic Research Needs for Geosciences: Facilitating 21st Century Energy Systems,” Office of Basic Energy Sciences of the U.S. Dept. of Energy, 2007; Jones & Maginn, 2010, ChemSusChem 3:863-864; Bara et al., 2010, Acc. Chem. Res. 43:152-159). This separation will become more important as world population expands and coal- and natural gas-fired electric power plants are increasingly utilized as sources of cheap electricity. In the natural gas industry, “sweetening” (i.e., CO2/CH4 separation) is a crucial process needed to obtain CH4 from natural gas wells with a degree of purity acceptable for piping, transport, and combustion (Sridhar et al., 2007, Sep. Purif. Rev. 36:113-174; Review of emerging resources: U.S. Shale gas and shale plays, 2011; available from: http://www dot eia dot gov). CH4 is also an incredibly important feedstock for the production of H2 via steam methane reforming (SMR) and the water gas shift (WGS) reaction (Descamps et al., 2008, Energy 33:874-881; Kohl & Nielsen, Gas Purification, 5th ed., Gulf Publishing Company, Houston, Tex., 1997; Barelli et al., 2008, Energy 33:554-570). The production of H2 in this manner is vital for the synthesis of chemicals such as NH3 and urea, as well as for clean energy applications (i.e., H2 fuel cells or combustion). Since CO2 is produced as an impurity in the SMR-WGS process, it must be separated and removed from the desired H2 product to generate sufficiently low levels for efficient H2 production (Descamps et al., 2008, Energy 33:874-881; Barelli et al., 2008, Energy 33:554-570). Ideally, the CO2 should be removed while leaving the H2 at high pressure and ready for transport or combustion. The development of separation technologies that can effectively and economically remove CO2 from these light gases is imperative for meeting the increasing regulations placed on CO2 emissions.
Polymer membrane-based gas separations have the potential to overcome many of the disadvantages associated with traditional CO2 separation technologies. Membrane processes have the advantages of scalability, small plant footprint, and ease of operation. However, in order for polymer membranes to be competitive with traditional separation methods they must possess both high CO2 flux and high CO2 selectivity (Zolandz & Fleming, Membrane Handbook, Chapman & Hall, New York, N.Y., 1992; Baker, Membrane Technology and Applications, 2nd ed., John Wiley & Sons Ltd., West Sussex, England, 2004). The range of polymer materials and separation performances for H2/CO2, CO2/CH4 and CO2/N2 separations are considerably large (Robeson, 2008, J. Mem. Sci. 320:390-400). Most polycondensation polymers, such as polycarbonates and polyimides, have been exhaustively studied for CO2/CH4 and CO2/N2 separations (Powell & Qiao, 2006, J. Mem. Sci. 279:1-49). CO2-selective polymer membranes for CO2/H2 separations (i.e., “reverse-selective”) are quite rare due to the typically high diffusion rate of H2 vs. CO2 (Zolandz & Fleming, Membrane Handbook, Chapman & Hall, New York, N.Y., 1992; Patel et al., 2003, Adv. Mater. 15:729-733; Lin et al., 2006, Science 311:639-642). Only a few examples of polymeric CO2-selective membranes for CO2/H2 separations exist in the literature, and include polyethylene glycol) (PEG) and its copolymers (Patel et al., 2003, Adv. Mater. 15:729-733; Lin et al., 2006, Science 311:639-642), which have exceptional CO2/light gas selectivities and CO2 permeabilities (Lin et al., 2005, Macromolecules 38:8381-8393; Lin et al., 2006, Adv. Mater. 18:39-44).
RTILs have been proposed as alternative “green” solvents to replace the volatile organic compounds (VOCs) typically employed in CO2 scrubbing (Baltus et al., 2005, Sep. Sci. Technol. 40:525; Anthony et al., 2005, Int. J. Environ. Technol. Manage. 4:105). RTILs can selectively permeate one gas over another (for example, CO2/CH4, CO2/N2, and CO2/H2; Rogers & Seddon, “Ionic liquids: Industrial applications for green chemistry,” Proceedings of the American Chemical Society meeting 2002, Washington, D.C.: American Chemical Society; Maase, “Industrial applications of ionic liquids,” in “Ionic liquids in synthesis,” 2008, Wiley-VCH Verlag GmbH & Co. KGaA. p. 663-687; Wilkes et al., “Introduction,” in “Ionic liquids in synthesis,” 2008, Wiley-VCH Verlag GmbH & Co. KGaA. p. 1-6), or separate products from a reaction mixture such as during a transesterification reaction (Hernandez-Fernandez et al., 2007, J. Mem. Sci. 293:73-80). Imidazolium-based RTILs are particularly attractive because of their distinctly superior CO2 solubility and separation properties compared to most other RTILs (Cadena et al., 2004, J. Am. Chem. Soc. 126:5300-5308; Scovazzo et al., 2004, J. Mem. Sci. 238:57-63; Bara et al., 2007, Ind. Eng. Chem. Res. 46:5380-5386).
Favorable CO2 solubility selectivity combined with “non-volatility” has led many researchers to investigate the performance of RTILs in a membrane configuration known as a supported ionic liquid membrane (SILM) (Scovazzo et al., 2004, J. Mem. Sci. 238:57-63; Morgan et al., 2005, Ind. Eng. Chem. Res. 44:4815-4823; Bara et al., 2009, Ind. Eng. Chem. Res. 48:2739-2751; Scovazzo, 2009, J. Mem. Sci. 343:199-211; Riisagera et al., 2006, Top. Catal. 40:91-102; Hanioka et al., 2008, J. Mem. Sci. 314:1-4; Myers et al., 2008, J. Mem. Sci. 322:28-31). Fabrication of a SILM is accomplished by saturating a non-selective, highly porous polymer support (e.g., poly(ether sulfone)) with a RTIL. Capillary forces alone are predominantly responsible for retention of the liquid RTIL component within the support. Employment of SILMs is attractive as RTILs possess negligible vapor pressures and can be impregnated into porous supports without evaporative losses—a hindrance for traditional supported liquid membranes (SLMs). For examples, imidazolium-based RTILs have been used in SILMs (Camper et al., 2006, Ind. Eng. Chern. Res. 45:6279-6283; Riisagera et al., 2006, Top. Catal. 40:91-102; Scovazzo, 2009, J. Mem. Sci. 343:199-211; Bara et al., 2009, Ind. Eng. Chem. Res. 48:2739-2751.). However, regardless of the nature of the liquid in the support (RTILs or others), the SLM configuration can fail if the pressure differential across the membrane is great enough to overcome capillary forces and push the liquid through the pores of the support. “Blow out” of the fluid RTIL component typically occurs at ≧1 atm of pressure drop. Various industrial gas separations occur at much higher pressures than SLMs can withstand, typically only a few atmospheres (Baker, 2002, Ind. Eng. Chem. Res. 41:1393-1411). Further, the thickness of the active RTIL separation layer in SILMs is limited to the porous polymer support thickness, which is typically 35-45 μm thick at minimum. For high filtration throughput in membranes, thinner active separation layers (≦1 μm thick) are desired because they have much greater permeate fluxes. Due to these limitations, SILMs are not at this time a widely viable technology for industrial membrane separations (Ferguson et al., 2007, Ind. Eng. Chem. Res. 46:1369-1374).
Solid-liquid composite materials formed from poly(RTIL)s and RTILs were developed (Bara et al., 2008, Ind. Eng. Chem. Res. 47:9919-9924; Bara et al., 2008, Polym. Adv. Technol. 19:1415-1420; Bara et al., 2009, Ind. Eng. Chem. Res. 48:4607-4610; Carlisle et al., 2010, J. Mem. Sci. 359:37-43; Carlisle et al., 2013, Ind. Eng. Chem. Res., 52; 1023-1032). These composite films contained 20 wt % RTIL and had enhanced liquid stability and superior CO2 separation characteristics compared to neat solid poly(RTIL) films. Further, CO2 permeability was improved by incorporating 45-75 wt % free RTIL in a cross-linked poly(RTIL) matrix (Carlisle et al., 2012, J. Mem. Sci. 397-398:24-37).
Very limited examples of step-growth imidazolium-based RTIL monomers and polymers have been described in the literature (Matsumi et al., 2006, Macromolecules 39:6924; Lee et al., 2011, Adv. Funct. Mater. 21:708; Carlisle et al., 2010, J. Membr. Sci. 359:37; Erdmenger et al., 2010, J. Mater. Chem. 20:3583; Williams et al., 2010, Polymer 51:1252; Amarasekara et al., 2012, Polymer Bull. 68:901). For example, imidazolium bis(epoxide) monomers and their polymers and cross-linked resins formed with amine monomers were reported (Demberelnyamba et al., 2004, Chem. Lett. 33:560; Paley et al., U.S. Pat. Appl. No. US 2010/0004389 A1; Libb et al., PCT Pat. Appl. No. WO 2010/002438 A1). The published methods for preparing imidazolium bis(epoxide) step-growth monomers from imidazole and highly reactive epichlorohydrin are either experimentally complex or do not generate pure and isolatable monomer (Paley et al., U.S. Pat. Appl. No. US 2010/0004389 A1; Libb et al., PCT Pat. Appl. No. WO 2010/002438 A1). In fact, the published reports fail to provide any characterization data on imidazolium bis(epoxide) monomers (Paley et al., U.S. Pat. Appl. No. US 2010/0004389 A1; Libb et al., PCT Pat. Appl. No. WO 2010/002438 A1), which is consistent with the fact that the monomers were never isolated and characterized.
Preparing a RTIL-containing thin film on the surface of a porous substrate is not trivial. RTILs have low molecular weights (i.e., they are very small compared to typical pore diameters) and are liquid at temperatures below 100° C. Consequently, RTILs diffuse much more readily into a porous substrate compared to a large polymeric molecule. It follows that, if a composition comprising a polymer and a RTIL is applied to the surface of a porous material, the RTIL penetrates the underlying porous material at a much higher rate than the polymeric molecule. Consequently, the film remaining on the surface of the porous substrate is depleted of or even free of RTIL, resulting in poor gaseous permeation characteristics (e.g., low permeance and/or low selectivity). An alternative method includes pre-coating the porous support with a “gutter layer.” A gutter layer is a film of dense polymer with high permeability and low selectivity. The gutter layer acts as a barrier to the penetration of the into the porous substrate. However, use of a gutter layer greatly reduces the overall gas permeance and selectivity of the composite membrane (i.e., reduces composite membrane performance)
There is a need for novel materials that may be used for selective membrane-based gaseous separations. Such materials should be easily and economically prepared, and should allow for the efficient separation of gaseous components from industrial gases in a timely fashion. There is also a need for novel methods of preparing thin, dense films composed of polymer and RTIL with improved gas permeability and selectivity. Such methods should allow for economical and efficient coating of a porous substrate, and the resulting composite membrane may be used for the efficient separation of gaseous components from industrial gases. The present invention addresses these needs.